Rare Sex and a Long Life Sustain Seaweed Populations at the Warm Edge of Their Range

ABSTRACT Aim The life cycle of many organisms is all but fixed across their distribution range. Most commonly, populations respond to environmental variation by shifting the timing of reproductive events (phenology). More profoundly, populations may (partly) shift their mode of reproduction from sexual to asexual. Life cycle variation can impact reproductive success, gene flow, genetic diversity and, ultimately, the evolutionary trajectory of populations. Understanding the factors that influence life cycle variation is essential for grasping the biology and ecological roles of species. This study investigates the variation in life cycles and its effects on the genetic diversity of a brown seaweed, Dictyota , across its European range. Location North‐East Atlantic Ocean and the Mediterranean Sea. Taxon Dictyota dichotoma (Hudson) J.V. Lamouroux (Phaeophyceae, Dictyotales). Methods We monitored phenology, fertility and lifespan in Atlantic and Mediterranean populations at its northern and southern boundaries and used microsatellite markers to assess how these factors influence genetic and genotypic diversity. Results We observed significant differences in phenology, reproductive strategies and genetic diversity among northern and southern European populations. In Mediterranean populations, D. dichotoma exhibited sporophytic dominance with gametophytes being extremely rare, suggesting a shift towards asexual reproduction. In contrast, North‐East Atlantic populations displayed more pronounced seasonal reproductive patterns with higher frequencies of gametophytes, indicating predominant sexual reproduction. However, genetic analysis showed lower allelic richness and unique alleles in northern populations, whereas southern populations were genetically more diverse, reflecting historical biogeographic processes. Clonal reproduction was more pronounced in the Mediterranean populations, influencing the spatial genetic substructure and contributing to locally lower genotypic diversity compared to Atlantic populations. Main Conclusions Our findings demonstrate that life cycle variation and phenology in D. dichotoma are closely tied to regional environmental conditions and have significant implications for population structure and genetic diversity. Results highlight how shifts in reproductive strategies contribute to the evolutionary and ecological dynamics of marine macroalgae across biogeographical gradients.